As you said you can YouTube Old Roman Chant and get a lot of recordings, mostly if not entirely from one group called Ensemble Organum. Of course, these are modern reconstructions. There were no recordings back then and only so much can be discerned from medieval sheet music. Chances are this chant did not sound exactly like these reconstructions, and the similarity to Eastern chant may be exaggerated by the necessity of fleshing out the music by imitating surviving forms of chant and the desire to provide a strong contrast to Gregorian Chant. And then there is simply the preferences of Ensemble Organum. Same goes for the other archaic forms of chant you will find on YouTube, Mozarabic, Ambrosian, Templar and so forth, again mostly from Ensemble Organum. They are sometimes, I think, based on rather flimsy evidence and a great deal of creative license. It’s too bad that a single group seems to dominate the field of archaic chant so strongly. I’d be interested in hearing different, independent takes on these forms.
Regarding the forms of chant themselves, I recall reading the following theory, though how solid it is I couldn’t say. The theory is that the Church in France at first used a Gallican chant very different from the chant used in Rome. When the Roman rite was imported to France it hybridized with the Gallican rite and Gallican chant hybridized with Roman chant. The name “Gregorian” chant may date to this period, when the hybrid chant, thought of in France as being Roman, was promoted against less Romanized forms of Gallican chant. Later it was imported from France into Italy, including Rome itself, where it seems to have existed alongside Old Roman Chant for several centuries. Perhaps the story, intended for Frenchmen, that the hybrid Gallo-Roman chant is the chant of St. Gregory was ironically part of why it eventually replaced Old Roman Chant entirely. Of course simple taste in music may also have played a role.
